Manufacturing analysis Polishing

of Sapphire Substrate



### FINAL PROJECT

POLISHING OF SAPPHIRE SUBSTRATES



## Contents

I Abstract (2)
1 Introduction (3) 1.1 Sapphire ( 3) 1.2 Sapphire Substrate ( 5) 1.3 Why do we polish ( 6 ) 1.4 Methods to polish sapphire substrate (6) 1.5 Process to manufacture sapphire substrate (6)
2 Application Sapphire substrate polishing process (8)
3 Polishing Methods and model (9) A. Polishing methods (9)
3.1 Mechanical Polishing (9)
3.2 Wet chemical–mechanical polishing (10) 3.3 Dry chemical–mechanical polishing (11) 3.4 Colloidal silica polishing (11) 3.5 Contactless polishing (12) 3.6 Two- step Chemical Mechanical Polishing (CMP) (13) B. Process model of sapphire substrates polishing (1 8 )
3.7 CMP model (1 8 ) 3.7.1 Dry-CMP (1 8 ) 3.7.2 Wet-CMP process mode (19) 3.8 Model of process variation for CMP of sapphire substrate (19) 4. Improvement of Sapphire Substrates Polishing Process (20 ) 4.1. Development o f PAD (20 ) 4.2. Development of slurry (21 )

III. Conclusions (22 )
IV.Discussion (22)
IV. References (23 )

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### I. Abstract : Single Crystal Sapphire is playing an ever-increasingly important role as a material for, high reliab ility Electronics today due to its excellent mechanical characteristics, chemical stability and light transmission. Products of sapphire have many applications in there substrate application for growth of another material on sapphire is very important. In fact the
substrate material demands stringent su rface quality requirements, that is, surface finish and flatness, are required. The use of CMP technique can produce high quality surface finishes at
low cost and with fast material removal rates. In final project, we have mentioned specification as well as processes to manufacture the sapphire substrate. After that process model for CMP process was presented. And finally we have also mentioned improvement of polishing process. Nowadays we have studied methods to improve polishing process such as using u ltrasonic flexural vibration to assist chemical mechanical polishing for sapphire
substrate
, chemical etching after CMP. H owever this project stated improvements of slurry and pad in CMP process. 

### II. Contents
1. Introduction:
1.1. Sapphire: DefinitionSapphire is a gemstone variety of the mineral corundum, an aluminium oxide (αAl 2 O 3 ). Sapphires are commonly worn as jewellery . Sapphires can be found naturally, by searching through certain sediments or rock formations, or they can be manufactured for industrial or decorative purposes in large crystal boule s. Structure Sapphire includes two types, nature sapphire (fig. 1b ) and synthetic s apphire (fig.1c ).

Page 2 Fig. 1.1a: Unit Cell of Sapphire Fig. 1.1b: Crystal structure of sapphire Fig. 1.1c: Synthetic sapphireManufacturing analysis Polishing

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Properties and applications Properties: Sapphire is the material of choice for engineers faced with the design challenges of extreme conditions such as those found in high-temperature, high-pressure or harsh chemical & physical environments. Its unique properties make it a cost-effective solution for those applications where long life and high performance are critical. One of the hardest and durable materials in existence, sapphire is virtually scratch proof which helps it maintain its integrity in demanding physical environments. It has low friction coefficient, excellent optical and dielectrical characteristics and a melting point of
over 2000 °C, making it ideal for high-temperature applications. It is chemically inert. It has the potential of delivering very high laser energies (>1 J/pulse), and easily withstands harsh chemicals such as fluorine plasma and other industrial gasses and fluids, with no particle generation. In addition sapphire can transmit ultraviolet, visible and infrared light was well as microwaves, a range broader than most materials. Tables 1, 2, 3 clearly show properties of sapphire.

Table1: P hysical properties of Sapphire
Name Metric English Chemical formula Crystal Structure
Unit Cell Dimension Al 2 O 3 Hexagonal System
a = 4.758A 0 , c = 12.991A 0
, Density
Hardness 3.98g/cc 1525-2000 Knoop, 9 mhos 0.144lb./in 3

Tensile Strength
At 20 0
At 500 0
At 1000 0 Flexural Strength Compression Strength 275 MPa to 400 MPa
400 MPa
275 MPa
355 MPa
450 MPa to 895 MPa 2.0 GPa (ultimate) 40,000 to 58,000 psi 58,000 psi 40,000 psi 52,000 psi 70,000 to 130,000 psi 300,000 psi Young’s Modulus, E Bulk Modulus, k Shear Modulus, G
MOR
Poisson’s Ratio 345 GPa
250 GPa
145 GPa
350 MPa to 690 MPa Sapphire is anisotropic 50 x 10 6 psi
36 x 10 6 psi
21 x 10 6 psi 50,000 to 100,000 psi It is orientation dependent

Table 2: Thermal properties of Sapphire

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Melting point Thermal Conductivity
At 0 0
At 100 0
At 400 0 Specific Heat
At 20 0 Heat Capacity
At 20 0
At 1000 0
Thermal Expansion
Coefficient
20 0 to 50 0
20 0 to 500 0 2310 K (2040 0 C)

46.06 W/(m.K) 25.12 W/(m.K) 12.56 W/(m.K)

0.187 cal/(g. 0 C)

0.187 cal/(mole. 0 C) 0.187 cal/(mole. 0 C)

5.8x10 -6 / 0 C
7.7x10 -6 / 0 C 3700 0 F

319.4 BTU in/hr ft 2 0 F
174.2 BTU in/hr ft 2 0 F
87.1 BTU in/hr ft 2 0 F

0.1827 BTU/lb 0 F

18.6 BTU/lb mole 0 F
29.9 BTU /lb mole 0 F

3.2 x 10 -6 / 0 F
4.3 x 10 -6 / 0 F

Table3: E lectrical properties of Sapphire

(frequency)
1.0MHz
3.0GHz
8.5GHz Volume Resistivity Dielectric Loss Constant tangent
9.39 0.0001
9.39 < 0.0001
9.39 < 0.00002
10 14 ohm.cm Dielectric Loss Constant tangent
11.58 0.0001
11.58 < 0.0001
11.58 < 0.00005

Applications: Due to these unique properties and wide optical transmission range (0.17 - 5.5 m) sapphire is used as the material for production of UV, visible and NIR optics for operation under critical conditions like high temperature, high pressure, chemically aggressive or
abrasive environment. In general speaking, Sapphire material has many applications such as in Jewelry Industry, in Engineering, in optics, in Medicine. In there, engineering application includes field of sapphire substrate that will be found in this final project. 

### 1.2

. Sapphire Substrates: Definition
So what is sapphire substrate? When manufacturer use s the sapphire as a substrate to fabricate other products, that means the sapphire used in the bottom of another materials.

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That sapphire is called sapphire substra te, for example Silicon on Sapphire Technology, and Silicon on Sapphire transistor. Properties Properties : Sapphire substrate are available in all orientations with the more common ones being R-plane (1-102), A-plane (11-20) also referred to as 90-degree Sapphire and Cplane (0001) referred to as 0-degree or basal plane Sapphire

, (please refer s to the above figure 1a). Table 4 states typical specification of sapphire substrate. As you can see that depending on types of sapphire substrate (dimension) we have different specification. However, only having some change parameters such as Outside Diameter, thickness, Orientation F lat, and Bow/Warp

. Table 4: Typical specification of sapphire substrate

Name Kyropoulos sapphire (Super sapphire)

Material High Purity and Monocrystalline Al 2 O 3 Purity Alumina purity 99.997%

Outside Diameter 50.8±0.2 mm and 76.2±0.2 mm

Surface orientation C-, A- and M-planes Off angle Plane, from 0.2 to 0.5” at 0.1 step

Thickness 420μm±10μm (Typical)

No. 1 orientation flat Length=22.0±2 mm (according to supply specs.)

No. 2 orientation flat Length=11.0±1 mm (for double-side polished) GBIR (TTV) <10μm

SOIR < 7μm Surface Roughness (Top side) Ra<0.15nm Surface Roughness (Rear side)

Bow/warp Ra<1μm (Same Ra for top side applies for doublesided products.) 0~ -10μm (for 2 ’’ sapphire substrate ), 0~ -15μm (for 4 ’’ ) Edge chamfering Rounded / Chamfering Angle (C) = 45 0 Laser Mark 8 characters, (TYMxxxxx) (T=TXT; Y=Year; M=Month; XXXXX=serial number ) marked in lapped surface, center aligned at OF, 1.6x0.8x0.6x1 mm (HxWxSxD)

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2 3 4 5 6 7 8 Slicing Cropping and Cylindrical Grinding Crystal growth Edge Beveling /Rounding Double-side lapping Mirror polishing Final cleaning Final inspection Manufacturing analysis Polishing of Sapphire Substrate



### 1.3. Why do we polish sapphire substrates? In many of the se applications critical surface quality demands of sapphire are required , that is
, surface finish and flatness are required. The generation of high-quality
surfaces with fine surface finish and low surface and subsurface damage is of critical importance. It has been established that the crystal structure of epitaxial films is strongly influenced not only by the substrate material and its orientation, but to a great extent also by the surface preparation of the substrate. Therefore, it is essential to use polishing techniques for polishing sapphire substrate. Reality shows that the polishing techniques may produce high quality surface finishes at low cost and with fast material-removal rates. 

### 1.4. Method s to polish sapphire substrates There are many methods to polish the sapphire substrate including, Mechanical polishing, Wet chemical–mechanical polishing, Dry chemical–mechanical polishing, Colloidal silica polishing, Contactless polishing CMP has been proved to be available method to produce high quality surface for sapphire substrate. 

### 1.5. Processes to manufacture sapphire substrates

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Fig 2: Processes to manufacture sapphire substrates Steps to fabricate the sapphire substrate as above, detail of processes are presented as
follow: 0. Orientation: The orientation for sapphire substrates is determined by two angles displaying a degree of difference at the surface or the direction of the ingot axis to the crystal lattice.
1. Crystal growth : By using Kyropoulos Method to grow sapphire ingot, crystal pulling 2. Cropping and Cylindrical Grinding : Both end of the grown crystal are cut in the direction of the intended angle to establish the ingot's surface orientation. The accuracy of the surface orientation is verified using an X-ray equipment. The diameter of the crystal ingot is adjusted to determine the wafer diameter by a grinding process. Through cylindrical grinding the ingot is made into a perfect cylindrical form ready for the following slicing process. 3. Slicing : The slicing procedure is critical because the way that the wafer is cut affects important qualities of the wafers such as thickness, taper and bow. To achieve maximum slicing efficiency and quality, the crystal ingot is sliced into wafers using the latest multi-wiresaw technology. In a wire saw, a single strand of steel thin wire moves from a feed to a takeup reel. In between, the wire wraps around three wire-guides containing hundred of guiding grooves that create a web of parallel wires. The ingot is fed together with abrasive slurry through the web to produce a concise cut with minimum kerf loss. 4. Edge Beveling /Rounding : Prior to processing the wafer surface, the edge of the substrate is beveled using the edge grinder. This process not only adjusts the wafer diameter to the precise specification, but also prevents edge chipping which may cause surface damage from loss fragments in the remaining manufacturing process. Page 7Manufacturing analysis Polishing

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5. Double-side lapping : Lapping is performed on both sides of the wafer. The primary purpose of lapping is to remove any irregularities on the wafers that may have occurred during slicing. The as-cut wafers are processed between two lapping plates using an abrasive slurry mixture to achieve first level of surface quality. Defects such as surface saw marks or wafer thickness variations are removed and corrected in this exercise. Once treated, the lapped wafers are ready for more precision. 6. Mirror polishing : In this polishing process wafer characteristics such as flatness, warp and thickness are fine toned to the precise measurements. For substrate used for SAW applications, the wafer flatness is of utmost importance. The polished surface must maintain the properties of single crystal and be free of scratch and digs and mechanical stress. Any crystal or processing defects remain on the wafer surface will impair the performance of the
SAW device. 7. Final cleaning : Any remaining particles and residues on the wafer surface are removed in a wet chemical cleaning process, after which the wafers are dried in a spin dryer before final inspection. 8. Final inspection : Various types of inspections occur throughout our entire manufacturing process to achieve highest product quality. During the final inspection, wafer characteristics are checked using the most advanced inspection equipment such optical microscope. At this point we make certain wafer quality in terms of its TTV, LTV, sori and bow either meet or exceed our customer's requirements. 

### 2. Applications of Sapphire Substrates Polishing Process Sapphire substrates belong to one of the most significant aspects of the constructional application of sapphire material. They are used for epitaxy of semiconductor films such as Si, GaN, AlGaN, and for making integrated circuits. Sapphire substrates are inert, work at high temperatures and mechanical loads, and can be obtained in large size. Therefore, they are used even in those cases when the lattice parameters do not completely coincide with the parameters of heteroepitaxial structures. In fact Sapphire product is serving mainly two applications: GaN-based LED and RF devices, both for mobile phones (Silicon-on-Sapphire “SoS” technology and Silicon on Sapphire
transistor )

. Sapphire single crystal combines many good mechanical and optical properties that make it become the choice of materials in a variety of modern High-Tech applications from commercial and military optical systems to high-power laser optics and high-pressure components, blue emitting diodes, laser diode devices, visible-infrared windows. In addition, the (0001) sapphire crystal wafers are an important substrate materials widely used in a range Page 8Manufacturing analysis Polishing

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of applications such as optics, electrics, semiconductor devices, integrated circuits industry and other applications. For convenience of positioning, the substrates are supplied with one or two additional profile planes : The C-plane is used for the coating of sapphire with CdS, CdTe, CdSe, GaN, SiC, InAlGaN, and LiNbO 3 as well as for the epitaxial growth of some oxide (e.g ZnO) andmetal films. The A-plane is used for making hybrid microcircuits, devices that possess hightemperature superconductivity, and for coating the crystal with Co, Fe ( 110) , W (110) , Au (111) , V (011) . The R plane is suitable for coating sapphire with MgO, α -ZnO, and Si by the method of heteroepitaxy. Sapphire substrates are also employed in sensors measuring pressure, mass, and humidity, as well as in IR-radiation detectors (HgCdTe films on sapphire) and other
devices. Among the promising trends in the use of sapphire substrates, one should mention the technology of carbon nanotube growth on sapphire. This new material seems to be promising
for nanotransistors and sensors. Researchers have found that a -plane (1120) sapphire surfaces spontaneously arrange single-walled carbon nanotubes into useful patterns. No template has to be provided to guide this structuring; it is formed automatically 

### 3. Polishing methods and model A. Polishing methods 3.1 Mechanical polishing It means this process only includes mechanical factors. The polishing material surfaces usually used are soft metals such as cast iron, tin, lead, or copper, and occasionally even various resins and plastics. These materials favor penetration of abrasive materials into the polishing surface. Therefore , only a part of t he abrasive grains work, and reduc e the load on
the treated su rface and removing the material (Fig 3. 1 ).

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Fig 3.1: Processing mechanism of mechanical polishing 

### 3.2 Wet chemical–mechanical polishing Its characteristic is using special substance to add to the polishing suspension, which activate the polishing process through chemical interactions with the workpiece surface. Normally
, al kaline SiO 2 solution (colloidal silica) is used for this purpose.As well as analogous polishing methods , proceed from the assumption that the chemical interaction of aluminum
oxide with silica is followed by the formation of an aluminosilicate:
Al 2 O 3 + 2SiO 2 + 2H 2 O→ Al 2 Si 2 O 7 ·2H 2 O The product of this reaction is disappered by friction force between the tool and workpice, this chemical reaction occurs at relative low temperatures. After this polishing process, they use an abrasive paste with a grain size of 1 μ m to get microasperities with average height
about 0.03 μ m. Meanwhile with CMP process using an Aerosil of SiO 2 micro-asperities do
not exceed 0.01 μ m. The earlier considered mechanisms of chemical–mechanical polishing

are complemented by a mechanism of chemical etching of the surface, which diminishes the incumbent defective layer. Figure 3.2 demonstrates the polishing factors essential for wet chemical–mechanical polish ing. For all referenced crystal the removal rate of material increases as specific pressure rises; note that sapphire polishing generally requires particularly high pressures (Fig. 3.2 ).

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Fig 3.2Wet-type chemical-mechanical polishing 

### 3.3 Dry chemical–mechanical polishing In this method, mechanical polishing is implemented through the action of chemical solidphase reactions between the abrasive and the sapphire workpiece . Surface porosity develops,
that enables removal of the outer layer of the crystal by soft polishers, and consequently
diminution of the surface-adjacent defective layer

. A typical example of this situation is polishing of sapphire by fine SiO 2 abrasive. When the SiO 2 particles interact with the sapphire surface, the area of contact undergoes local reactions of high pressure and temperature. Therefore , solid-phase reactions proceed between the SiO 2 and the Al 2 O 3 ; the sapphire surface becomes porou s and the reaction products are carried away by a soft polisher easilly

. T he efficiency of sapphire polishing by colloidal silica is not high in the presence of water. Dry polishing using SiO 2 gel is more effective, it requires high temperatures and protection of the working zone from sputtering of the gel. The polishing tool used has a specially designed working surface and to constantly kept in silocozole suspension. This arrangement provides continuous supply of the reagents to the polishing zone. I nto the range
of 0.02 mg/min, t he efficiency of such a polishing progresses ≈ 40 times higher than the procedure of polishing on a quartz faceplate with a colloidal silica water solution (0.0005 mg/min). Using these methods , we can get a sufficiently high surface quality to be obtained, but the problems of stress and crystal lattice distortions are remain. 

### 3.4 Colloidal silica polishing Colloidal silaca polishing method is based on from colloidal phenomenon that arises
when atomically small particles of silica (10–100 Å) are used in a slurry of preset alkalinity

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(example pH 4-12). Very fine colloidal silica is supplied to a soft polisher, and pressure is applied to the workpiece, a gelling phenomenon peculiar to colloidal solutions starts working

. (Fig. 3.3 )

Fig 3.3 Constitutional diagram of colloidal silica polishing In wet and dry CMP method which we arready have studied before a certain extent depend on the chemical properties of the workpiece surfaces, thereby can be effectively applied to a limited set of mat erials.But colloidal silica polishing method is applicable to practically all materials and provides achievement of mirror like surfaces free from stresses (no residual stress). 

### 3.5 Contactless polishing

This method also can be called elastic emission machining (EEM) and results in reduction of material removal by a factor of 10–100 in comparison with mechanical polishing. Particles with a diameter of about 100 Å interact with the workpiece surface, removing only several
tens of atoms

.

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Fig 3.4 Principle of elastic emission machining Fig 3.4 shows the “polishing float” phenomenon, when the distance between the polishing facility and the workpiece surface approaches values on the atomic or molecular order, the slurry particles and the surface atoms will join together

. By this mechanism way , separation of the adjoined particles from the workpiece surface does not cause plastic flow. Hence , the
surface did

not was damaged as these particles are removed. Note that the treatment efficiency ( MRR ), the surface quality, the thickness or absence of
the defective layer, and the precision and accuracy of sur face shape are interrelated. Obviously
, CMP i s characterized by a relatively high treatment efficiency and insignifi cant resultant defective layer. However, more important role of chemical reactions, more difficulty

to controlling process precision. The utilization of soft polishing surface materials raises the surface quality, but deteriorates surface shape control. 

### 3.6 Two- step Chemical Mechanical Polishing (CMP) In generally, CMP process requires high removal rate and low surface roughness. However it is so difficult to meet these requirements by a single step polishing process. To
obtain an ultrasmooth surface of the sapphire substrate, we have to investigate two-step CMP of the sapphire substrate . First step, we us ed ultrafine α -alumina-based slurry and nanoscale silica-based slurry for second step. Preparation of α -alumina-based slurry :

α - alumina-based slurry includes: C alcined α

– alumina abrasives have different particle shape s with an average diameter of 500 nm (as shown in Fig 3.5) and a bulk density of 0.8

g / cm 3 . The 5 % wt

alumina powder and 0.5% wt sodium hexametaphosphate, which act as a dispersant, were added to deionized ( DI ) water in a container under stirring. Then using 0.1 M potassium hydroxide solutions to adjust the solution pH = 12. Finally, the mixture was
filtrated with a 20 μ m pore strainer. Preparation of silica-based slurry : Include Macrogol 6000 (0.5%wt) as a surfactant, 5 % wt silica gel self-made with an average diameter of 50 nm (as shown in Fig 3.6) in a container under stirring. and using triethanolamine to adjust the solution pH=12. Finally, the mixture was filtrated with a 1 μ m pore strainer.

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Figure 3. 5 SEM image of alumina particles. Figure 3. 6 SEM image of silica Polishing tests : U sing a CMP tester ( CETR, CP-4 ) to polish s apphire wafers [(0001) oriented] .The parameters of polishing process are given in table 5

. Process conditions The first step The second step Pad rotation speed (rpm) 100 100 Wafer rotation speed (rpm) 100 100 Down force (psi) 5 5 Slurry feed rate (mL/min) 100 100 Polishing time (min) 60 30 Polishing pads Polyuretanes pad Politex pad

Table 5 : Process parameters of CMP process
Characterization methods
We use Hitachi S-4700 field-emission

- scanning electron microscope to investigate t he morphology of the abrasive particles . Using HCl/KOH to adjust the desired value pH (we
need pH 12 because at this value material removal rate (MRR) is highest)

.

MRR = 107×∆mρ×2.542×π×t ( nm/min )
➢ ∆ m ( g ) is the mass variation in sapphire before and after polishing
➢ t ( min ) is the polishing time
➢ ρ is the density of sapphire
The surface topography and root-mean-square ( rms ) roughness w as measured by a Quesant Q-Scope 250 atomic force microscopy ( AFM ) . The AFM operating mode was the contacting
mode, and the scan area was 10 10 μ m 2 . The MRR and rms roughness is the average of 3

individual polishing tests.

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### Results Optimization of process parameters : T he polishing parameters such as down pressure and rotation speed have an important influence on the CMP performance . F or the second step, any little change in polishing parameters may have a strong effect on polished surface quality .The influence of polishing pressure and rotation speed ( both wafer and pad ) on the MRR and rms roughness in the
second step using a silica-based slurry, and the results are shown in Fig. 3.7 and 3.8

.

COF (coefficient of friction ) analysis

Page 15 Fig 3. 7 : Effect of polishing pre ssure on the MRR and rms roughness

Fig3.8: Effect of rotation speed on the MRR and rms roughnessManufacturing analysis Polishing

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The friction force is dependent on interfacial electrostatic interactions, dynamic surface conditions, properties of the opposing surfaces, and the abrasive size, which all influence the

contact area between the opposing surfaces. T he COF in the second step is larger than that in the first step. Because the polishing pad used in the second step is the soft pad that make
increase contact area between the pad and surface of the sapphire substrate

.

Fig 3.9: COF as a functional of polishing time for two steps

In this reasearch, the sapphire supstrates polished were ground wafers and has many rough peak (fig 3.10) .These rough peaks were first removed during the polishing process. When polishing time increase, the number of rough peaks decreased. Therefore, we have result like fig3.10.When the rough peaks are completely removed, the contact area tends to be constant
and COF is stable.

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Fig 3.10 Representative AFM images from center, middle, and edge of the sapphire indifferent CMP stages: (a) Before CMP, (b) after the first-step CMP, and (c) after the secondstep CMP. 

### MRR and rms analysis

Before polishing After the first step After second step RMS (R 0 ) 968.9 21.98 6.83 MRR (nm/min) 42.3 7.1

Table 6 : RMS and MRR of sapphire substrates in different CMP stages
From table we see that t he rms roughness value of the sapphire surface is very high before polishing. After the first-step CMP using Al 2 O 3 slurry, the rms value decreased from 968.9 to 21.98 Å. L ess than to 6.83 Å after second step using SiO 2 slurry with the optimized process parameters. It means that the subnanometer precision sapphire surface obtained by using the two-step CMP. T he first step gives higher MRR but poor surface quality, meanwhile the second step gives good surface quality but lower MRR. The first-step CMP is suitable for

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## b

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preliminary polishing to provide high polishing rate as well as full surface planarization, whereas the second-step is suitable for final polishing to provide a fine local planarization

. CMP mechanism The two kinds of slurry used in this method were composed of different abrasive particles with different shapes, hardness, and size, which result in separate removal rate and surface roughness. The Mohs hardness of α -alumina and silica are 7 and 9, and the sapphire has the
same hardness as α -alumina

. 

### B . Process model o f sapphire substrates polishing

3.7 CMP model In general, CMP process can be illustrated like this schematic diagram.

Fig 3.11 schematic diagram of CMP process 3.7.1 Dry-CMP

Page 18 Polishing machine & other equipments Pad/Slurry/ Sapphire substrate Sapphire substrate Grits & material Pad & Grits Force friction model & Passivation Model Abrasive Abrasion ModelManufacturing analysis Polishing

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Parallel process with chemical passivation and mechanical abrasion

Fig 3.12 process model of dry-CMP

Page 19 Polishing machine & other equipments Pad/Slurry/ Sapphire substrate Sapphire substrate Grits & material Pad & Grits Hydrodynamic model & Passivation Model Abrasive Abrasion ModelManufacturing analysis Polishing

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Fig 3.13 Process model of Wet-CMP
This CMP process has lot of parameter, but it can be sumarized in this table



### inputs outputs

Pad: Fiber structure conditioning, compressibility modulus Material removal

Wafer geometry and material Surface quality : roughness, scratching

Slurry: pH, oxidizes, buffering agents, abrasive concentration, abrasive geometry and
size distribution Within - wafer non-uniform
material removal

Process: Pressure , velocity , temperature, slurry flow, polishing time Within- die non-uniform material
removal

Table 7: Parameters of CMP process
3.8 M odel of process vari a tio n for CMP of sapphire substrate

Ei(t)

In CMP process of sapphire subtrate, we have these parameters: es: Spindle velocity, down force, tool path, chemical reaction, temperature, passion layer
thickness. Page 20 Polishing machine & other equipments Polishing machine & other Sapphire substrate Ei(t)

es,ep ms,
mp 

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ep: Pad stiffness, grit hardness chemical slurry. ms: Passivation, wafer surface, stiffness, temperature material, total thickness variation. mp: Material chemical, material hardness. From this model we can have variation equation. This equation shows affection of disturbances, input into out put(Y). Resulting of output depends on α



### 4. Improvement of Sapphire Substrates Polishing Process

Introduction: To gain smooth surface sapphire wafer, there are many ways to do it, but I concentrate on improvement PAD and Slurry. 

### 4.1. Development of PAD: The latest VISIONPAD polishing pads are VISION PAD 6000 and VISION PAD 5200[1] , which are enormous improvements not only increasing removal rate but also reducing consumable cost. 1) VISION PAD TM 5200 polishing pads are presented next-generation technology which reaches a high removal rate for Tungsten (W), ILD and Cu bulk processes. Thanks to implement unique polymer chemistry and more pad porosity, it gains from 10 percent to 30 percent rise in removal rate for W, ILD and Cu Bulk applications. As a result of growth removal rate, customer can save polishing times and slurry consumption to dramatically lower CMP consumable cost. In addition, VISIONPAD 5200 polishing pads achieve from 10 to 20 percent cut in shortcoming of W and Cu applications, also improve dishing and corrosion in W applications over Dow’s standard IC 1000 TM polishing pads. 2) VISION PAD TM 6000 polishing pads have some features: low-defect, low-hardness polymer chemistry and optimized pore size, so it can exceed the removal rates of the IC
1000 TM polishing pads. As a result of testing customer, VISIONPAD TM 6000 can cut down a 50 to 60 percent scratch defects, while the disc defects (Dishing) decreased by 35%, and the wafer non-uniformity and the IC1000 pad level is very

. 

### 4.2. Development of slurry:

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a) Thanks to two-Step Chemical Mechanical Polishing of Sapphire Substrate [2], researchers have an ultra smooth surface of sapphire wafer. The first state, when they use alumina-based slurry, the first time the coefficient of friction declines and then it tends to be a

constant. At the first step, the root-mean-square (rms) roughness value of the polished surface can reduce from 968.9 to 21.98 Å. In the second state, As a result using the nanoscale silicas slurry, at the first minute the coefficient of friction grew after that became unchanged and the rms roughness can drop at about 6.83 Å. b) With using mixing abrasive slurries [3], which contain boron carbide and ceria abrasive, has removal rate about 180 nm/min and gain a root mean square (rms) about 2.1 nm. More Over, reactive ion etching process, we can improve the quality sapphire wafer with
rms to 0.7nm.

This plot shows AFM images of the sapphire substrate before and after CMP. In fig (a) and (b), with only boron carbide abrasives, the rms reduces from 297.2 to 19.9 nm after CMP, but deep scratches remain obvious because of the direct fierce physical tear of the hard boron carbide particles. In Fig. (c), using typical MAS for CMP, the surface quality greatly rise with an rms of 2.1 nm due to ceria particles’ interaction with the sapphire hydration layer and a fierce indirect physical tear of the hard B4C core. However, the surface is still smooth. From fig (c), we can see shallow scratches and inescapable particle and dust bond existence. To improve the surface quality, we have an RIE process. We have used a established Oxford Plasma lab 80 Plus RIE system. With a radio frequency (13.56 MHz) glow discharge creates this plasma. Electronic grade CF4 and Ar gases were injected into the chamber through mass flow controllers with the flow rates of 10 standard cubic centimeters per minute (sccm) and Page 22Manufacturing analysis Polishing

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40 sccm, respectively. After 20 min etching, the rms has dramatically dropped to 0.7 nm. From Fig. d, a finely global planarization of the wafer has been gained. In this RIE process,
we have a formula:
2Al 2 O 3 (s) + 3CF 4 (g) → 4AlF 3 (s) + 3CO 2 (g) (4) The product of AlF3 is not evaporative (bp 1276°C, 1 mm at 1238°C), but the ability of reactive ions to etch a given compound greatly depends on the boiling points and vapor pressures of potential products during etching. Low boiling points and high vapor pressures of potential products will push the etching process, so the RIE performance can be attributed to a weak chemical reaction between CF4 and sapphire surface and a gentle physical sputtering by Ar gas, for which impurities adherence and the bulge of the sapphire surface can be etched. Because of these reasons, a super surface of sapphire was obtained after the RIE process. 

### III. Conclusions Thanks to model process of CMP sapphire substrate, we can control variations to have desire output product. Also, the improvement of Sapphire wafer plays an important role in reducing cost of sapphire substrate and improving quality it.
IV.Discussion Depend on the aim of different applications; we have the different requirement of sapphire wafers quality. Therefore, we can choose the suitable polishing process. The limit aspect of sapphire substrate is that we can not know exactly disturbances and experments.



### IV. References [1] http://content.yudu.com/A1pft8/Nanotimes09-2010/resources/26.htm? skipFlashCheck=true [2 ] Liu, Weili ; Song, Zhitang ; Hu, Xiaokai
, two-step chemical mechanical polishing of sapphire substrate, Published May 3, 2010 [3] Liangyong Wang,z Kailiang Zhang, Zhitang Song, and Songlin Feng, chemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Wafer, (2007) Page 23Manufacturing analysis Polishing

of Sapphire Substrate
[4] Elena R. Dobrovinskaya, Leonid A. Lytvynov, Valerian Pishchik Gavish, Sapphire, (2009) [5] Wenhu Xu, Xinchun Lu *, Guoshun Pan, Yuanzhong Lei, Jianbin Luo, Ultrasonic flexural vibration assisted chemical mechanical polishing for sapphire substrate, 25 January
2010 [6] Kurlov V.N
. , Kiiko V.M. , Kolchin A.A. , Mileiko S.T.J. Cryst. Growth. 204 , 1999 , 499 [7] Zhukov L.F., Litvinov L.A., Chugunnyi E.G. Patent USSR. 766237, 1980 [8] Zhukov L.F., Litvinov L.A., Shumikhin V.S. Patent USSR. 1256572, 1984 [9] Ivanina B.M., Litvinov L.A., Priimach B.S., Globus M.E. Patent USSR 1316467, 1985. [10] Schewe P
. , Riordon J. , Stein B. Phys. News Update . 1 , 2003 , 619 .

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